Solid-state lighting fixtures, such as those using light emitting diodes (LEDs) are not yet widely used for general illumination. Nonetheless, solid-state lighting technology is rapidly evolving, and more powerful LEDs are being released every 6-12 months. Though very energy efficient, LEDs are just now being developed with sufficient efficacy (i.e., light output for unit of energy input, or lumens per Watt) to enable lighting systems based on one or a small number of LEDs. As a result to produce sufficient illumination in most applications, prior art solid-state lighting systems utilized many LEDs, such as clusters of LEDs arranged on printed circuit boards in arrays and mounted to large, heavy, and expensive heat sinks. However, if an LED malfunctioned it was not efficiently replaceable. Most LEDs could not be unscrewed simply and replaced as with other forms of lighting, such as incandescent, fluorescent, high-pressure sodium, metal halide, high-intensity discharge, and others. Furthermore, as newer, brighter, higher efficacy LEDs came on the market, the entire prior art LED array required replacement, and likely a complete heat sink redesign, because the system was most often constructed as a single integrated unit. Today, few modularized lighting systems are available that allow for upgrades to the newest LED technology without developing new components for the rest of the system.
In addition, LED lighting fixtures are extremely efficient in terms of both power consumption and reliability. In economic terms, operating costs for LED fixtures are approximately one-tenth the cost of standard incandescent or tungsten-halogen lamps and just one hundredth the cost of natural gas powered lamps. Furthermore, LED light sources can operate 50,000-100,000 hours depending on conditions compared to thousands of hours for conventional light sources. Consequently; there are tremendous economic advantages to providing LED-based lighting fixtures that consume much less energy with greatly reduced maintenance costs. Given their power efficiency, LEDs are uniquely capable of using solar-powered rechargeable batteries to operate separate from the electrical power grid. However, because the amount of available solar power varies by location and from day to day due to weather variability, the charging of the batteries and the powering of the LEDs must be careful controlled to provide a reliable light fixture.
Directing and controlling the light output from an LED is also a challenge. Most prior art systems used either no optics, preferring to simply aim arrays or individual LEDs where light was desired, or used individual lens caps (known as total internal reflection lenses or TIRs) mounted directly over each LED. However, these prior art methods generally resulted in a series of over-illuminated hot spots of increased intensity light surrounded by darker rings of lower intensity. These non-uniform light distributions represented wasted energy since an efficient lens design can effectively spread the light from a hot spot out over a larger area, thereby improving safety and nighttime security. Moreover, with recent dark sky compliance requirements and regulations directed to decreasing light pollution becoming a new criteria for outdoor municipal and commercial lighting systems, control of light becomes a more critical issue.
Finally, there is an important aesthetic aspect to the conversion to solid-state lighting systems. Consumers have very strong preferences for light fixtures that resemble the prior art systems with which they are familiar. Maintaining these familiar form factors prevents the use of prior art TIRs in most cases and generates additional difficulties for efficiently directing light in a useful distribution for a given application. Therefore, there is a need for a modular LED light engine that can be utilized in many different applications, is adaptable to the continuing improvements in LED efficacy and construction, controls the light output effectively and efficiently, and has an aesthetically appealing form factor. Further, there is a need for a solar-powered lighting fixture utilizing LEDs with automatic control of light output to adjust power consumption to the corresponding solar illumination available for recharging the lighting fixture.